You can see exactly how 1H NMR signals fall into these regions in the following collection of spectra. The first two spectra each contain only one peak because every proton in benzene and in cyclohexane is identical. In benzene the peak is at 7.5 ppm, where we expect a proton attached to an unsaturated C atom to lie, while in cyclohexane it is at 1.35 ppm because all the cyclohexane protons are attached to saturated C atoms. Again, to help comparisons, we have also included the 13C spectra of benzene and cyclohexane. For benzene, the signal falls in the unsaturated C region (100–150 ppm), at 129 ppm, while for cyclohexane it is in the saturated C region, at 27 ppm.

tert-Butyl methyl ether is a solvent and fuel additive whose ¹H spectrum illustrates the effect of a nearby oxygen atom: the large peak at 1.1 ppm comes from the nine H atoms making up three identical methyl groups of the tert-butyl part of the molecule, while the three H atoms of the methyl part of the ether are at 3.15 ppm. These three hydrogen atoms are all bonded directly to a C atom, which itself is bonded to O, whose electronegativity attracts their electrons, deshielding the 1H nuclei and shifting them to larger chemical shift.

The plane of symmetry we noted in the 13C NMR spectrum of BHT means that the 1H NMR spectrum of the related compound Topanol 354 is relatively simple for a compound with 26 H atoms: a large peak and two small peaks between 5 and 0 ppm for the 18 protons of the tert butyl groups and the three protons of each methyl group, and another small peak between 5 and 10 ppm for the two protons attached to the aromatic ring.

¹H NMR has many more features, which we will leave aside for the moment, and it is no exag -geration to say it is in general more important for the routine determination of structure than all the other methods put together. We will come back to 1H NMR in more detail in Chapter 13.

مواضيع ذات صلة

A guided tour of the 13C NMR spectra of some simple molecules

Different ways of describing chemical shift

Regions of the 13C NMR spectrum

The reference sample—tetramethyl silane, TMS

The chemical shift scale

Why do chemically distinct nuclei absorb energy at different frequencies?

NMR also uses radio waves

NMR uses a strong magnetic field

Nuclear magnetic resonance What does it d

Atomic composition can be determined by high-resolution mass spectrometry

Mass spectrometry detects isotopes

Mass spectrometry by chemical ionization, electrospray, or other methods